Powered subwoofers often switch into standby mode when there's no low-frequency information coming in after a certain period of time. When the program material does contain low-bass information, the sub switches back into active mode.

As for what the receiver's owner's manual says, it might be helpful if you tell us which receiver it is and what, specifically, the owner's manual is saying.

If the outlets on the back of the receiver are labeled "switched" than whatever is plugged into them will turn on and off with the receiver.

Those outlets on the back of the receiver are desined to power source components (cd/dvd etc). A subwoofer amplifier would draw more current than one of those outlets could provide, and will eventually shut down.

Anything with an amplfier should be plugged into its own outlet. (a powerstrip is usually okay, exept in some high end installations)

Amps, subs, other receivers, TVs shouldn't be plugged into the receivers power outputs (anything that will draw a lot of power). A DVD player, CD player or the like can though.

The auto on feature in your sub is triggerred by the line level signal. It will have some threshhold. If the level in the line is below that threshhold the auto on circuit will trip the sub off. Options are to turn the sub to on all the time (what I do). Turn the sub trim on your receiver up a bunch and reduce the gain knob on the sub plate amp to compensate (might solve the problem). Or live with it.

It is the Sony DE985, actually I didn't read it right, it says: Connect the AC power cords of your audio/video components to a wall outlet. If you connect other audio/video components to the AC outlets on the receiver, the receiver will supply power to the connected components allowing you to turn the whole system on or off when you turn the receiver on or off.